1. Technical Field
The present disclosure generally relates to memory systems and in particular to scheduling of operations in memory systems. Still more particularly, the present disclosure relates to controlling scheduling of memory access operation and high latency memory operations in memory systems.
2. Description of the Related Art
One of the highest latency operations performed in a memory is a refresh operation. Refresh operations are periodically performed in a dynamic random access memory (DRAM), which requires periodic refresh operations in order to retain the contents of one or more memory banks. Without constant refreshing, a DRAM will lose the data written to the DRAM as memory cell capacitors leak their charge. DRAM manufacturers and standards committees have defined a maximum interval or time period between refresh operations (tREFI). A DRAM is refreshed responsive to a refresh command periodically issued by a memory controller. The refresh operation takes a time period called a refresh cycle time (tRFC) to complete. The refresh cycle is completed before the memory banks being refreshed can be accessed by a scheduled read operation.
Technological advancements have led to an increase in the capacity (or density) of DRAM chips. As the DRAMs capacity increases, so to does the refresh cycle time for each of the ranks containing the DRAMs. For low density DRAM chips, the refresh cycle time has a negligible effect on read performance. However, for high density DRAM chips, memory reads can be required to wait until the required refresh operation is completed. This long wait time can result in degraded performance for the high density DRAM chip. For example, a 1 gigabyte DRAM can have a refresh cycle time of 110 nanoseconds, while an 8 gigabyte DRAM can have a refresh cycle time of 350 nanoseconds. In contrast, a read operation to a memory location can typically be completed in 25 nanoseconds. As future DRAM chips are built with higher densities and increasing capacity, the refresh cycle time is projected to increase further.